1.1 Field of the Invention
The invention relates to the field of molecular biology; in particular, to immunogenic compositions and recombinant VMP-like genes useful for treatment and diagnosis of Lyme disease. Also included are methods for the determination of virulence factors in Lyme disease.
1.2 Description of Related Art
Lyme disease is a bacterial infection caused by pathogenic spirochetes of the genus Borrelia. The infection can occur in humans, dogs, deer, mice and other animals, and is transmitted by arthropod vectors, most notably ticks of the genus Ixodes. Borrelia burgdorferi, the most common cause of Lyme disease in North America, was first cultured in 1982. B. garinii and B. afzelii are the most common infectious agents of Lyme disease in Europe, and another species, B. japonicum, has been described in Japan. These organisms are closely related and cause similar manifestations with multiple stages: an expanding rash at the site of the tick bite (erythema migrans); fever, lymphadenopathy, fatigue, and malaise; effects of disseminated infection, including carditis, meningoradiculitis, and polyarthritis; and chronic manifestations including arthritis and neurologic disorders. Lyme disease is often difficult to diagnose because of shared manifestations with other disorders, and it can also be refractory to treatment during late stages of the disease. It is most common in areas such as suburban regions of upstate New York and Connecticut, where large populations of deer and white-footed mice serve as the principal mammalian hosts and reservoirs of infection. Approximately 10,000 cases of Lyme disease in humans are reported per year in the United States, and it is also a significant veterinary problem due to a high infection rate of dogs and other domestic animals in endemic regions.
B. burgdorferi, the etiologic agent of Lyme disease, is able to persist for years in patients or animals despite the presence of an active immune response (Steer, 1989; Schutzer, 1992). Antigenic variation has been postulated previously as a mechanism whereby B. burgdorferi evades the immune response in the mammalian host (Schwan et al., 1991; Wilske et al., 1992). Antigenic variation has been defined as changes in the structure or expression of antigenic proteins that occurs during infection at a frequency greater than the usual mutation rate (Borst and Geaves, 1987; Robertson and Meyer, 1992; Seifert and So, 1988).
Relapsing fever is another disease caused by pathogenic Borrelia. It has both epidemic and endemic forms. The epidemic form is caused by B. recurrentis and is transmitted between humans by lice. It was a major source of morbidity and mortality during World War I, but has been rare since then due largely to public health measures. Endemic relapsing fever is an epizootic infection caused by several Borreliae species, including B. hermsii. It occurs sporadically among hunters, spelunkers, and others who come in contact with infected soft-bodied ticks of the genus Ornithidorus. Relapsing fever is characterized by two or more episodes or “relapses” of high bacteremia (up to 108/ml). The first wave of infection is caused by Borreliae expressing a certain Variable Major Protein (VMP) on their surface (e.g. Vmp21). The gene encoding this VMP is located at a promoter site in the expression plasmid, whereas over 24 nonexpressed copies of different VMP genes are present on the so-called silent plasmid. When the host develops antibodies against the expressed VMP, the organisms of that stereotype are destroyed and the patient improves. However, a small proportion of organisms have undergone antigenic switching to a different stereotype. Nonreciprocal recombination occurs between the expression plasmid and the silent plasmid, resulting in the insertion of a different VMP gene in the expression site (e.g., Vmp7). The organisms expressing Vmp7 are not affected by the anti-Vmp21 antibodies, and therefore multiply in the host and cause a second episode of the disease. Up to five of these 3–5 day episodes can occur, separated by 1–2 week intervals.
Such well-demarcated episodes of infection do not occur during Lyme disease, and fewer organisms are present in the blood and in tissues at any stage. However, there are reasons to suspect that similar mechanisms of antigenic variation may occur in B. burgdorferi and other Lyme disease Borreliae. The infection, if untreated, commonly persists for months to years despite the occurrence of host antibody and cellular responses; this observation indicates effective evasion of the immune response. Lyme disease may be disabling (particularly in its chronic form), and thus there is a need for effective therapeutic and prophylactic treatment.
Certain B. burgdorferi genes and proteins have been patented, including Outer Surface Protein D (OspD) (U.S. Pat. No. 5,246,844; issued Sep. 21, 1993). OspD has not proven to be a useful protein for diagnosis or immunoprotection. Other proteins, including OspA and OspC, have been considered as vaccine candidates for Lyme disease, including a recombinant OspA vaccine currently in human clinical trials. Other vaccines are in use or undergoing testing in veterinary applications, including vaccination of dogs. However, animal studies indicate that OspA vaccination may not be effective against all strains of Lyme disease Borreliae. OspA is also not useful for immunodiagnosis, due to weak antibody responses to OspA in Lyme disease patients.
Previous studies have generally failed to provide evidence for the occurrence of antigenic variation in Lyme disease Borrelia. Genetic heterogeneity in the genes encoding the membrane lipoproteins OspA, OspB, OspC, and OspD has been well documented among strains of Lyme disease Borreliae (Marconi et al., 1993; Marconi et al., 1994; Livey et al., 1995). In addition, mutations in ospA and ospB have been shown to occur in vitro (Rosa et al., 1992; Sadziene et al., 1992). However, no significant antigenic change (Barthold, 1993) or gross genetic alteration (Persing et al., 1994; Stevenson et al., 1994) has been detected in B. burgdorferi N40 isolates from chronically infected BALB/c and C3H mice, other than the loss of the 38-kilobase (kb) plasmid encoding OspD. Therefore the heterogeneity in Osp proteins observed among B. burgdorferi sensu lato isolates appears to represent evolutionary divergence (“antigenic drift”) rather than antigenic variation.
There is a commercial demand for vaccines and diagnostic kits for Lyme disease, both for human and veterinary use. Several companies have active research and development programs in these areas.